"Other moons usually grab the spotlight, but it turns out Mimas is more bizarre than we thought it was," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena. "It has certainly given us some new puzzles."

Scientists expected smoothly varying temperatures, peaking in the early afternoon near the equator.

Instead, the warmest region was in the morning, along one edge of the moon's disk, making a sharply defined Pac-Man shape, with temperatures around 92 Kelvin. The rest of the moon was much colder, around 77 Kelvin. A smaller warm spot - the dot in Pac-Man's mouth - showed up around the Herschel crater.

The warm spot around Herschel makes sense because the three-mile-high tall crater walls can trap heat. But scientists were completely baffled by the sharp, V-shaped pattern.

"We suspect the temperatures are revealing differences in texture on the surface," said John Spencer, a Cassini composite infrared spectrometer team member. "It's maybe something like the difference between old, dense snow and freshly fallen powder."

Even if surface texture variations are to blame, scientists are still trying to figure out why there are such sharp boundaries between the regions, Spencer said. It is possible that the impact that created Herschel Crater melted surface ice and spread water across the moon. That liquid may have flash-frozen into a hard surface.

But it is hard to understand why this dense top layer would remain intact when meteorites and other space debris should have pulverized it by now, Spencer said.

"These processes are not unique to Mimas, but the new high-definition images are like Rosetta stones for interpreting them," Paul Helfenstein, a Cassini imaging team associate, said.